This invention relates to a diffraction grating in which a plurality of parallel grooves each having asymmetrical triangular cross-sectional configuration are provided in the major surface of a single crystalline silicon substrate in a predetermined direction and in a closely spaced relationship with a predetermined period or pitch, and a method manufacturing such diffraction grating structure.
Such diffraction grating structure has been used, for an example, as a demultiplexer for a wavelength-division-multiplexing optical fiber transmission system.
According to a prior art method of manufacturing such diffraction grating structure, for the purpose of readily manufacturing at high reproduction-ability a plurality of the diffraction gratings, a ruling machine has been used to prepare a master with a plurality of ribs corresponding to the grooves of the diffraction grating, the master being used for production of replicas.
According to this method, however, it is necessary to use an expensive working machine, that is, the ruling machine and to rely upon high degree of technique to manufacture the master, thus increasing the cost of the diffraction gratings.
Where many types of diffraction gratings are to be manufactured, it is necessary to prepare a plurality of masters having different pitches and different cross-sectional configuration of the ribs.
As one approach to this problem, a method has been proposed in which a single crystalline silicon is subjected to an anisotropic or preferential etching process as disclosed in Won-Tien Tsang and Shyh Wang paper entitled "Preferentially etched diffraction gratings in silicon", Journal of Applied Physics, Vol. 46, No. 5, May 1975. According to this method, however, the cross-sectional shape of the groove is a symmetrical equilateral triangle or a symmetrical trapezoid so that it is impossible to freely select the blaze angle, one of the important factors of a diffraction grating.
One method of forming grooves having asymmetrical triangular cross-sectional configuration by the suitable selection of the etching initiation surface and anisotropic etching process is disclosed in a paper of Jorg Muller entitled "Ultrafast Multireflection-and Transparent Thinfilm Silicon Photodiodes", International Electron Devices Meeting 1976, Technical Digest 17.6. According to this method, the asymmetrical triangular grooves are contiguous to a metal substrate, and light energy transmitting through the photodiode and reaching the metal substrate is reflected at reflection surfaces along the asymmetrical grooves a number of times so as to increase the efficiency of the photodiode. However, the grating manufactured by this method is not constructed to provide angular dispersion to the light.
Furthermore, a method of manufacturing grooves having asymmetrical cross-sectional configuration in a GaAs substrate by the anisotropic etching technique is disclosed in a Shyh Wang et al Paper entitled "Preferential Chemical Etching of Blazed Gratings in [110]--oriented GaAs", Optics Letters, Vol. 4, No. 3, March 1979.
In anisotropic etching of silicon substrate, the etching speed in &lt;111&gt; direction (normal to {111} plane) is slower than that in any other directions, whereas in a GaAs substrate, the relative etching speed R.sub.hkl of the crystal surface {hkl} is R.sub.{110} .gtorsim.R.sub.{111}B .gtorsim.R.sub.{100} &gt;&gt;R.sub.{111}A thus there are two etching speeds namely R.sub.{111}B and R.sub.{111}A with reference to the {111} plane. For this reason, when the etching initiating plane (11N) (where N=0 to 3) is subjected to the anisotropic etching process as disclosed in this paper, grooves having symmetrical triangular configuration can be formed for etching initiation planes of (110), N being zero and (111) N being one, but for the etching initiating planes of (112) N being two and (113), N being three, only grooves having asymmetrical triangular grooves having definite angles of inclination can be obtained. For this reason, this method can not be applied to diffraction grating structures in which it is essential to set any desired blaze angle according to the pitch (grating constant) and blaze wavelength.